Liposomes have been used as carriers for therapeutic agents, where antibodies may be conjugated to the liposome to provide specific targeting of the liposome/agent complex to a diseased cell or tissue. (See, e.g., Xu et al., Mol. Cancer. Ther., 1:337-346 (2002); Torchilin, et al., Proc. Nat'l. Acad. Sci., 10: 6039 (2003); U.S. Pat. No. 6,165,440; U.S. Pat. No. 5,702,727; U.S. Pat. No. 5,620,708; U.S. Pat. No. 5,565,215; U.S. Pat. No. 6,530,944; U.S. Pat. No. 6,562,318; U.S. Pat. No. 6,558,648; and U.S. Pat. No. 6,395,276). The antibody or antibodies of choice for targeting purposes may bind to a known tumor associated antigen (TAA) or to an antigen associated with infectious diseases or other disease states.
One such tumor-associated antigen is CD74, which is an epitope of the major histocompatibility complex (MHC) class II antigen invariant chain, Ii, present on the cell surface and taken up in large amounts of up to 8×106 molecules per cell per day (Hansen et al., 1996, Biochem. J., 320: 293-300). CD74 is present on the cell surface of B-lymphocytes, monocytes and histocytes, human B-lymphoma cell lines, melanomas, T-cell lymphomas and a variety of other tumor cell types. (Hansen et al., 1996, Biochem. J., 320: 293-300) CD74 associates with α/β chain MHC II heterodimers to form MHC II αβIi complexes that are involved in antigen processing and presentation to T cells (Dixon et al., 2006, Biochemistry 45:5228-34; Loss et al., 1993, J Immunol 150:3187-97; Cresswell et al., 1996; Cell 84:505-7).
CD74 also plays an important role in cell proliferation and survival. Binding of the CD74 ligand, macrophage migration inhibitory factor (MIF), to CD74 activates the MAP kinase cascade and promotes cell proliferation (Leng et al., 2003, J Exp Med 197:1467-76). Binding of MIF to CD74 also enhances cell survival through activation of NF-κB and Bcl-2 (Lantner et al., 2007, Blood 110:4303-11).
Murine LL1 (mLL1 or murine anti-CD74 antibody) is a specific monoclonal antibody (mAb) reactive with CD74. Cell surface-bound LL1 is rapidly internalized to the lysosomal compartment and quickly catabolized, much faster than other antibodies, such as anti-CD19 or anti-CD22 (Hansen et al., 1996, Biochem. J., 320: 293-300). LL1 was reported to exhibit the highest rate of accumulation inside B cells of any of the antibodies tested (Griffiths et al., 2003, Clin Cancer Res 9:6567-71).
Murine LL1 was developed by fusion of mouse myeloma cells with splenocytes from BALB/c mice immunized with preparations from the Raji B-lymphoma cell line (called EPB-1 in Pawlak-Byczkowska et al., Can. Res., 49: 4568 (1989)). The clinical use of mLL1, just as with most other promising murine antibodies, has been limited by the development in humans of a human anti-mouse antibody (HAMA) response. A HAMA response is generally not observed following injection of mLL1 Fab′, as evidenced in a bone marrow imaging study using an mLL1 Fab′ labeled with 99mTc. Juweid et al., Nuc. Med. Comm. 18: 142-148 (1997). However, in some therapeutic and diagnostic uses, a full-length anti-CD74 antibody may be preferred. This use of the full-length anti-CD74 antibody can limit the diagnostic and therapeutic usefulness of such antibodies and antibody conjugates, not only because of the potential anaphylactic problem, but also as a major portion of the circulating conjugate may be complexed to and sequestered by the circulating anti-mouse antibodies. Although the use of antibody fragments of mLL1 may circumvent the problems of immunogenicity, there are circumstances in which whole IgG is more desirable and the induction of cellular immunity is intended for therapy or enhanced antibody survival time. In general, HAMA responses pose a potential obstacle to realizing the full diagnostic and therapeutic potential of murine anti-CD74 antibodies. Therefore, the development of immunoconjugates that include chimeric, humanized and human anti-CD74 binding molecules, (e.g., antibodies and fragments thereof, antibody fusion proteins thereof, multivalent and/or multispecific antibodies and fragments thereof), would be extremely useful for therapy and diagnosis, with reduced production of human anti-mouse antibodies.